1. Field of the Invention
The present invention relates to a positive type cutting insert which is detachably fitted to an outer peripheral portion of a tool such as a face milling cutter and a method of manufacturing the cutting insert.
2. Discussion of the Background
As a positive cutting insert of this kind (hereinafter referred to simply as "insert"), one such insert is that disclosed in Japanese Utility Model Laid-Open No. 63-136815 is known.
This insert has an insert body having a parallelogrammic plate-like shape and has cutting edges formed on a pair of ridge portions of an upper surface of the insert body and a pair of ridge portions of a lower surface of the insert body in a staggered position relative to the upper surface ridge portions in the direction along the periphery of the upper or lower surfaces. A rake is formed on each of side surfaces of the insert body connecting to the cutting edges. The rake is inclined toward a center of the upper or lower surfaces of the insert body away from the cutting edge in the direction of thickness of the insert body.
The entire surface of the rakes, connecting to the opposite pairs of the cutting edges of the upper and lower surfaces of the insert body, is formed of a convex curved surface such that the distance between the pair of rakes connecting to each opposite pair of cutting edges is larger at the centers of the cutting edges than at each of the leading and trailing ends of the cutting edges. That is, each of the rakes is formed by being curved along the periphery of the upper or lower surface having the corresponding one of the cutting edges on the ridge portion so as to round out in an outward direction from the insert body with such an inclination so as to be closer to a center of the lower or upper surface at a position closer to the lower or upper surface. Accordingly, the corresponding cutting edge is formed along a convex curved line, and a positive radial rake angle is set for the cutting edge.
The thus-arranged cutting insert is attached to a tool body of a cutting tool such as a face milling cutter in such a manner that one of the upper and lower surfaces is positioned as a flank on an outer circumferential side of the tool while the other is brought as a seated surface into contact with a bottom surface of an insert attachment seat, with one rake positioned so as to face in the tool rotating direction. Cutting is performed with the cutting edge positioned on the intersection ridge portion between this rake and the upper or lower surface that serves as a flank.
In the insert having the above-described construction, since each side surface is formed as a convex curved surface, the intersection angle between the side surface as a rake and the side surface (front end flank) disposed so as to face the tool front end side can be increased, thereby improving the strength of the insert at the intersection portion to prevent chipping or excessive wear. Further, since the cutting edge is formed so as to have a convex curved surface, the axial-direction rake angle of the cutting edge is gradually increased from the leading end to the trailing end of the cutting edge at the time of chamfering, thereby ensuring good cutting quality.
When the above-described insert is attached to the tool body, the two side surfaces other than the side surface set as a rake and the side surface set as a front end flank are brought into contact with a surface of the insert attachment seat facing the tool rotating direction and another surface facing the tool front end side, whereby the cutting edge is accurately positioned.
In the conventional insert, however, each of the side surfaces to be brought into contact with these surfaces of the attachment seat is entirely formed as a convex curved surface. Accordingly, it is possible that the insert body will be shifted relative to the insert attachment seat by sliding the curved surfaces on the seat to change the position of the cutting edge, resulting in a reduction in the accuracy of cutting with the cutting tool.
Ordinarily, this insert is designed or manufactured in such a manner that the shapes of the side surfaces formed as rakes are set on the basis of reference surfaces formed as flat-plane surfaces on the insert, and the positions at which the cutting edges are formed are determined on ridge portions in which the side surface and the upper and lower surface intersect each other. In the conventional insert, however, only the upper and lower surfaces serve as such reference flat-plane surfaces, and there is a need to form the rakes as convex curved surfaces. Therefore, it is difficult to accurately form the rakes and cutting edges in the predetermined positions. Consequently, it is possible that errors will occur with respect to the shapes of the rake and the cutting edges. Because of this possibility and the above-described possibility of a shift of the insert, there is a risk of the cutting accuracy being further reduced.
The accuracy of the cutting edges and the rakes of a manufactured insert is ordinarily measured on the basis of the flat surfaces of the insert. However, there is also a possibility of a hindrance with respect to accurate measurement of the conventional insert for the same reason.